This invention relates to marine power distribution systems for supplying drive power of variable frequency and variable voltage for a ship propulsion motor as well as fixed frequency standard voltage power for ship service loads.
Conventional integrated marine electric drive systems generate AC power at 60 Hz and either utilize the AC power directly at 60 Hz, through frequency converters for ship's propulsion motors and transformers for ship's auxiliary power circuits, or rectify the AC power from the generator and distribute direct current (DC) power and then convert the DC power to AC power using conversion equipment for the ship's propulsion motors and also for the ship's auxiliary loads.
The commercial industry as well as the military has many years of experience generating and distributing power at 50/60 Hz. Conventional 50/60 Hz generators operate at speeds of 1200 rpm up to 3600 rpm. This is shown below and in Table 1.
Frequency: ##EQU1##
TABLE 1 Limitation of 50/60 Hz Generators rpm Number of Poles Frequency 3600/3000 2 60/50 1800/1500 4 60/50 1200/1000 6 60/50
The size and weight of a generator are inversely related to the generator speed and the generator pole number in the following manner; Size.varies.1/(rpm.multidot.poles) and Weight.varies.1/(rpm.multidot.poles). Consequently, Size and Weight.varies.1/(rpm.multidot.poles). Therefore, to provide a higher power density, resulting in a reduction of size and weight for a given power level, a higher generator frequency (&gt;60 Hz) must be provided by using a higher generator speed (&gt;3600 rpm) and/or a higher pole number (&gt;2).
The use of a higher frequency to increase power density has been employed successfully by 400 Hz distribution systems in the aircraft industry. However, no such application has been shown to be feasible in conjunction with 60 Hz (or other AC or DC power) for both service power and vehicle propulsion in marine vessels. Since higher power density is important for marine applications, a generator for such applications should produce high frequency power as a result of either an increased generator speed, a higher pole number (&gt;2) or a combination of both.
Transformer power density, i.e. size or weight per unit power, is affected by many variables. The main variables which will influence size and weight are: (1) specified sound level requirements (both airborne and structureborne); (2) operating electrical frequency; (3) operating voltage; (4) voltage impulse and dielectric levels; (5) phase shifting requirements; (6) efficiency; (7) type and configuration of cooling system (air, water, forced); and (8) winding configurations, e.g. arrangement of multiple windings. As the required power levels increase the motor drive input transformer weights make up a larger percentage of the overall system weight. Studies have shown that transformers have an average power density of 3.5 lbs/hp for large-scale motor power conversion equipment. However, depending on system requirements, the transformer power density has been estimated in the range of 2.3 lbs/hp to 5.6 lbs/hp. Normally, to achieve the best power densities in the range of 2 lbs/hp, higher operating frequencies and forced cooling of some type must be used. When supplying ships with shaft horsepower from 25,000 shp to 100,000 shp or higher the transformer weight becomes significant. Table 2 provides an indication of transformer weights at various shaft horsepower levels and power densities.
TABLE 2 Transformer Weight versus Shaft Horsepower Transformer Transformer Weight (lbs) Weight (lbs) Shaft Horsepower (Power Density 2.3 (Power Density 5.6 Level lbs/hp) lbs/hp) 25,000 57,500 140,000 50,000 115,000 280,000 80,000 184,000 448,000 100,000 230,000 560,000
In the Satterthwaite et al. U.S. Pat. No. 4,661,714, an electric marine propulsion system includes two diesel engines connected to drive corresponding alternators at a speed which is two or three times the normal operating speed of the alternators, thereby producing power at a frequency two or three times the standard 60 Hz frequency, i.e. from 120 Hz to up to 180 Hz. The generator outputs are supplied through a ship service switchboard to a series of frequency converters which supply ship propulsion motors with power at frequencies which may vary from 0 to 60 Hz. In addition, the switchboard supplies high frequency power to a transformer and frequency converter for the ship's service utility and auxiliary power at standard 60 Hz frequency.
The Levedahl et al. U.S. Pat. No. 5,684,690 discloses an electric power supply system for propulsion and service control in which each engine turbine drives both a service alternator and a separate propulsion alternator.
The patent to Limpaecher, U.S. Pat. No. 5,270,913, relates to a transformerless semiconductor power conversion system in which a series of H-bridges are used to convert AC power to high voltage DC power. Similarly, the Bansal et al. U.S. Pat. No. 4,780,569 uses H-bridge rectifiers receiving power from a plurality of AC generators with phase-shifted output windings supplying AC power to isolated H-bridge rectifier arrangements to produce a high voltage DC output, while the patent to O'Brien, Jr., U.S. Pat. No. 4,114,555, discloses a power supply for a marine vessel in which a plurality of diesel motor-AC generator units are connected in parallel to provide AC power that is rectified and voltage-controlled by a plurality of SCRs supplying DC motors, thereby avoiding the use of transformers in a propulsion motor power supply.
In integrated electric plants of the prior art, limitations of frequency, voltage, and generator arrangement are presented. Such limited integrated electric power distribution systems have certain disadvantages with respect to power density, i.e. size and weight.